1. Field of the Invention
This invention relates generally to a technique for determining the relative humidity of a cathode input airflow to a fuel cell stack and, more particularly, to a technique for determining the relative humidity of a cathode input airflow to a fuel cell stack that includes determining water flow through a water vapor trap unit that humidities the cathode input airflow.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane defines a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The membranes within a fuel cell stack need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. If the wetness of the membranes is not at or near an optimum level, then the durability of the membranes is reduced. Therefore, to help maintain membrane relative humidity, it is known in the art to humidify the cathode airflow to the cathode side of the stack.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust from the stack will include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust, and use the separated water to humidify the cathode airflow input.
FIG. 1 is a schematic diagram of a fuel cell system 10 that humidifies a cathode input airflow to a fuel cell stack 12 in this manner. A compressor 14 provides a compressed airflow on line 16 to the cathode side of the stack 12. A humidified cathode exhaust gas is provided on line 18 at the output of the cathode side of the stack 12. The airflow from the compressor 14 on the line 16 is directed through one side of a WVT unit 20 and the cathode exhaust gas on the line 18 is directed through another side of the WVT unit 20. The WVT unit 20 includes permeation membranes or other porous materials, as is well understood in the art, that collects water vapor and liquid water in the cathode exhaust gas and uses this water to humidify the airflow to the cathode input.
The relative humidity of the cathode airflow and the pressure within the fuel cell stack 12 need to be tightly controlled for proper fuel cell stack operation. The mass flow of the water transferred through the WVT unit 20 depends on the partial pressure of the water, the flow of air through the WVT unit 20 and other system parameters. To control the relative humidity and pressure, proportional control valves 22 and 24 are selectively opened and closed to direct the cathode exhaust gas through the WVT unit 20 or by-pass the WVT unit 20 on a by-pass line 26.
A dew-point sensor 28 measures the relative humidity of the cathode airflow into the stack 12 on line 16, and that value in combination with the stack operating conditions, such as current density, temperature, pressure, etc., determines the position of the control valves 22 and 24 so that the proper relative humidity is provided for the cathode input airflow. A first temperature sensor 30 measures the temperature of the airflow in the line 16 and a second temperature sensor 32 measures the temperature of the cathode exhaust gas in the line 18. A pressure sensor 34 measures the pressure in the inlet line 16, and a mass flow meter 36 measures the flow of air from the compressor 14. These values and other system operating parameters are used to control the operation of the system 10, as is well understood in the art.
As mentioned above, the dew-point sensor 28 determines the relative humidity of the cathode input airflow. However, the known dew-point sensors used for this purpose have a number of disadvantages that affect the ability to accurately determine the relative humidity of the cathode input airflow. Particularly, dew-point sensors that provide the required accuracy are not standard in automotive applications. Further, these sensors are relatively expensive, have low reliability and require analog control inputs. Also, the known dew-point sensors have problems with transient behavior and do not provide accurate readings if a droplet of water forms on the sensor contacts. It is therefore desirable to provide a different technique for determining the relative humidity of the cathode input airflow that does not require a sensor.